Polyphosphazene membrane separation of polar from non-polar fluids

ABSTRACT

Processes utilizing polyphosphazene membranes for separating polar fluids from non-polar fluids are disclosed. Fluid separation membranes comprised of polyphosphazenes having halogenated side groups are disclosed as exhibiting effective preferential selectivity and permeabilities for polar fluids relative to non-polar fluids contained in feedstream mixtures. The polyphosphazenes are in general rubbery polymers having attractive fluids transport properties and improved thermal and chemical stability.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 566,243 filedDec. 28, 1983 (now abandoned).

BACKGROUND OF THE INVENTION

This invention relates to fluid separation membranes comprised ofpolyphosphazene polymers and processes utilizing such membranes forselectively separating fluids from fluid mixtures by permeation. Inanother aspect the invention relates to fluid separation membranescomprised of polyphosphazene polymers and copolymers having halogenatedside groups which exhibit preferential selectivity and permeabilitiesfor polar fluids relative to non-polar fluids contained in a feedstreammixture.

The separating, including upgrading or the concentration of at least oneselective fluid from a fluid mixture, is an essentially importantprocedure in view of demands on the supplies of chemical feedstocks.Frequently these demands are met by separating one or more desiredfluids from fluid mixtures and utilizing the product for processing.Applications have been made employing separation membranes forselectively separating one or more fluids from fluid mixtures. Toachieve selective separation, the membrane exhibits less resistance totransport of one or more of the fluids than of at least one other fluidin the mixture. Thus, selective separation can provide preferentialdepletion or concentration of one or more desired fluids in the mixturewith respect to at least one other fluid and, therefore, provide aproduct having a different proportion of the one or more desired fluidsto at least one other fluid than the proportion in the feed mixture.However, in order for selective separation of the one or more desiredfluids by the use of separation membranes to be commercially attractive,the membranes must satisfy several criteria so that the use of theseparation procedure is economically attractive. For instance, themembranes must be capable of withstanding the conditions to which theymay be subjected during the separation operation. The membranes alsomust provide an adequately selective separation for one or more desiredfluids at a sufficiently high flux, that is, permeation rate of thepermeate fluid per unit surface area. Thus, separation membranes whichexhibit adequately high selective separation but undesirably low fluxes,may require such large separating membrane surface area that the use ofthese membranes is not economically feasible. Similarly, separationmembranes which exhibit a high flux but low selective separation arealso commercially unattractive. Accordingly, work has continued todevelop fluid separation membranes which can provide both an adequatelyselective separation of one or more desired fluids, for example, polarfluids from non-polar fluids at a sufficiently high flux for an extendedperiod of time under adverse environmental conditions such that the useof these fluid separation membranes is economically feasible.

In general, the passage of a fluid through a membrane may proceedthrough pores, i.e. continuous channels for fluid flow in communicationat both feed and exit surfaces of the membrane which pores may or maynot be suitable for separation by Knudsen flow and diffusion; in anothermechanism, in accordance with current views of membrane theory thepassage of a fluid through the membrane may be by interaction of thefluid with the material of the membrane. In this latter postulatedmechanism, the permeability of a fluid through a membrane is believed toinvolve solubility of the fluid in the membrane material and thediffusion of the fluid through the membrane. The permeability constantfor a single fluid is presently viewed as being the product of thesolubility and diffusivity of the fluid in the membrane. A givenmembrane material has a particular permeability constant for passage ofa given fluid by the interaction of the fluid with the material of themembrane. The rate of permeation of the fluid, i.e. flux, through themembrane is related to the permeability constant, but is also influencedby variables such as membrane thickness, partial pressure differentialof the permeate fluid in gaseous form across the membrane, thetemperature and the like.

Polymers useful as practical membranes for fluid separation applicationsmust satisfy a number of stringent criteria. Foremost among requirementsare the polymers intrinsic transport properties such as permeability andselectivity. Additional requirements include adequate thermal andchemical-environmental stability and appropriate properties such assolubility characteristics which are crucial to the fabrication of thepolymer into useful membranes. At present, most polymers which have beenutilized for fluid separations, belong to the general family known asglassy polymers. For the most part, these materials are attractivebecause they satisfy very well the above criteria for fabrication intouseful membranes in the asymmetric morphology either as film or hollowfiber. However, many polymers which satisfy fabrication criteria possesstransport properties which are less than ideal for a given separationapplication. Frequently polymers which exhibit a desirably highselectivity for a particular gas pair or solute-solvent pair do notallow the faster species to permeate at an adequate rate. Coversely,polymers with very high permeabilities for a given permeant, often areonly moderately selective. lt is a difficult task to find a singlematerial which will simultaneously satisfy most or all of the necessaryrequirements for the desired fluid separations.

Glassy polymers are genrally highly amorphous materials, which are, astheir name implies, in a frozen state at ambient temperatures. Above theglass transition temperature or T_(g) of the polymer, the glass solidchanges into another amorphous solid state, a rubber which then ischaracterized by much more rapid motion on the molecular scale of thepolymer chains. Of particular interest among the various propertieswhich distinguish polymers in the rubbery state versus the glassy stateis that the transport properties are often drastically different for thetwo types of materials. Permeabilities for gases through many rubbersare very high compared to permeabilities of the same gases in manyglassy polymers. However, the more dynamic nature of the polymer chainsin the rubbery state, which is generally responsible for the higherpermeabilities, often causes much lower selectivities for rubberypolymers as compared to many glassy polymers. In addition, many rubberypolymers do not possess an appropriate combination of other propertiesrequired for efficient fabrication into membranes having the preferredasymmetric morphology.

Some rubbery polymers have been and are being used in gas separations.Silicone rubbers have been applied to air (O₂ /N₂) separations,particularly for small scale uses such as blood oxygenation or airoxygen enrichment. In such a circumstance, it is the very high O₂permeability of silicone rubbers which outweighs less attractiveproperties, such as low selectivity and mechanical weakness. Since thesilicone rubbers cannot readily be made in asymmetric form, the polymeris supported typically on a relatively strong porous support. Suchporous supports can in appropriate applications effectively circumvent arubbery polymer's limitations regarding fabrication and mechanicalstrength. For large scale gas separation applications of potentialcommercial importance, it remains, however, that the usually inadequateselectivity characteristics of most rubbery polymers limit theirpractical utility.

Polyphosphazenes are polymers having a phosphorous-nitrogen sequencewith organic substituents on the phosphorous as follows: ##STR1## whereR and R' are the same or different organic substituents and n is aninteger of ten or more.

A limited number of single gas transport measurements ofpolyphosphazenes has been made. For instance, Bittirova, et al,Vysokomol. Soedin, Ser. B, 23(1), 30-3 (1980) discloses the permeabilityto oxygen, nitrogen and argon of poly(octyloxy phosphazene). TheBittirova, et al reference focuses on one polyphosphazene withparticular interest in the material because of its "specific properties"including the translucent, flexible, elastic films having permeabilitycoefficient values for O₂, Ar, N₂ of 12.84×10⁻⁷, 11.88×10⁻⁷ and5.25×10⁻⁷, cm³.-cm/cm².s.atm, respectively. The reference makes noattempt to qualify the elastic films further with regard to other gastransport properties or mixed gas separations.

Kireyev et al, Vysokomol. Soedin, Ser. Al8(1), 228 (1976) andChattopadhyay, et al, J. Coating Technology, 51 (658), 87 (1979)disclose water vapor permeability in poly(butyloxy phosphazene) and inpoly(aryloxy phosphazenes) respectively. Kireyev, et al discusses theneed for new types of elastomers; thus, the interest in polydiorganophosphazenes (one of the qualifying physical property studies relates tothe absorption of steam by these phosphazenes as examinedgravimetrically). Chattopadhyay, et al provides a publication entitled"Polyphosphazenes As New Coating Binders" with special interests in thepolyaryloxy phosphazenes as a material having a high degree of flameretardancy and other desirable polymeric properties for application aspaint binders. The Chattopadhyay, et al reference along with otherphysical test evaluations indicate moisture vapor transmission throughthe polymeric film at 25° C. No mention of separation of gas or fluidmixtures by polyphosphazene membranes has been made.

In summary, suitable polyphosphazene fluid separation membranes have notbeen provided. Particularly, the suitability of polyphosphazene fluidseparation membranes for separatlon of polar fluids from non-polarfluids has not been suggested.

SUMMARY OF THE lNVENTION

The present invention provides fluid separation membranes comprised ofpolyphosphazenes. Polyphosphazenes having halogenated side groups arealso provided which exhibit effective preferential selectivity andpermeabilities for polar fluids relative to non-polar fluids containedin feedstream mixtures. The polyphosphazene fluid separation membranesare in general comprised of rubbery polymers having preferentialselectivity for fluid transport for one or more fluids in a mixed fluidfeedsteam and improved thermal and chemical stability. Polyphosphazeneswhich are suitable according to the present invention are polymers whosebackbone consists of phosphorous nitrogen sequence with organicsubstituents on the phosphorous: preferred constituents being alkoxy,aryloxy and substituted alkoxy and substituted aryloxy. The phosphazenefluid separation membranes can have various configurations or be in theform of a dense film. Due to the rubbery nature of polyphosphazenemembranes, suitable fluid separation membrane structures will oftenutilize the polyphosphazenes as a supported coating on a poroussubstrate. In addition, polyphosphazenes can be applied as coatingmaterial in contact with a porous separation membrane which contributesto the separation properties of the resulting multicomponent membrane.The present invention is also directed to processes utilizingpolyphosphazene membranes for separating fluids from fluid mixtures.

DEFlNITlON OF TERMS

In the description of the present invention the following definitionsare used.

The term "fluid" as used in this application means a form of matter thatcannot permanently resist any shearing force which causes flow such as:a gas, e.g., a conditon of matter in which the molecules flow apparentlywithout resistance; and a liquid, e.g., a condition of matter in whichthe molecules move freely but are restrictive by gravitation. The termfluid can also include a mixture of gases and liquids.

The term "polar fluid" as used in this application means those fluidscapable of ionizing or functioning as an electrolyte as opposed to anonelectrolyte or fluids characterized by such properties as relativelyhigh dielectric constants or molecular dipole moments.

The term "membrane" as used in this application refers to materialhaving surfaces which can be contacted with a fluid and/or gas mixturesuch that one fluid or gas of the mixture selectively permeates throughthe material. Such membrane can generally be disposed in film- or hollowfiber- form. Membranes can be porous, or essentially pore-free, or havelayers that are porous and layers that are essentially pore-free. Thisinvention provides membranes exhibiting advantageous fluid separationproperties for polar fluids. However, the membranes of this inventionwill exhibit useful fluid and/or gas separation properties other thanfor polar fluids.

The term "dense", or "dense film", membranes as used in this applicationmeans membranes which are essentially free of pores, i.e., fluidchannels communicating between surfaces of the membrane, and areessentially free of voids, i.e. regions within the thickness of themembrane which do not contain the material of the membrane. Since adense membrane is essentially the same throughout the structure, itfalls within the definition of isotropic membrane. Although some thickdense membranes are very selective, one of their disadvantages is lowpermeate flux due to the relatively large thicknesses associated withmembranes. Dense membranes are useful in determining intrinsic gasseparation properties of a material. Intrinsic separation propertiesinclude separation factor α, and permeability constant, P, both of whichare defined below.

The term "asymmetric" or "anisotropic" membranes as used in thisapplication means membranes which have a variable porosity across thethickness of the membrane. Exemplary of an asymmetric membrane is whatis called the Loeb membrane, which is composed of two distinct regionsmade of the same material, that is, a thin dense semi-permeable skin anda less dense, void-containing support region. However, an asymmetricmembrane does not necessarily have the thin dense semi-permeable regionon an outer surface or skin.

The membranes of this invention comprise materials in film- or hollowfiber- form which have particular separation relationships. Some ofthese relationships can conveniently be stated in terms of relativeseparation factors with respect to a pair of gases for the membranes. Amembrane separation factor (αa/b), for example, for a given pair ofgases (a) and (b) is defined as the ratio of the permeability constant(P_(a)) for the membrane for a gas (a) to the permeability constant(P_(b)) of the membrane for gas (b). The permeability for a given gas isthe volume of gas at standard temperature and pressure (STP), whichpasses through a membrane per square centimeter of surface area, persecond, for partial pressure drop of one centimeter of mercury acrossthe membrane per unit of thickness, and is expressed in units of P=cm³-cm/cm² -sec.-cmHg. A separation factor is also equal to the ratio ofpermeability (P/1)_(a) of a membrane of thickness (1) for a gas (a) of agas mixture to the permeability of the same membrane for gas (b),(P/1)_(b) (P/1=cm³ /cm² -sec-cmHg).

In practice, the separation factor with respect to a given pair of gasesfor a given membrane can be determined employing numerous techniqueswhich provides sufficient information for calculation of permeabilityconstants or permeabilities for each of the pair of gases. Several ofthe many techniques available for determining permeability constants,permeabilities and separation factors are disclosed by Hwang, et al"Techniques of Chemistry, Vol. VlI, Membranes In Separations, John Wiley& Son (1975) (herein incorporated by reference) at Chapter 12, pages296-322.

"Polyphosphazenes" as used in this application represent a compositionof matter having a repeating structural unit of the formula: ##STR2##where Y and Y40 can be the same or different and selected from oxygen,nitrogen or sulfur and where R and R' can be the same or different andselected from substituted alkyl, aryl, and substituted aryl, alkyl, andn is an integer of from about 100 to about 70,000. Preferredpolyphosphazenes are alkoxy, substituted alkoxy, aryloxy and substitutedaryloxy groups wherein the R and R' can be the same or different andcontain from 1 to about 25 carbon atoms.

As an example, substituted polyphosphazenes as used in this applicationrepresent a composition of matter having a repeating structural unit ofthe formula: ##STR3## where n is an integer of from about 100 to about10,000 or more, ORX and OR'X' each represent a halogenated alkoxy orhalogenated aryloxy where R and R' can be the same or different withfrom 1 to about 25 carbon atoms, X and X' can be the same or differenthalogens.

The term "crosslinked polymer" as used in this application means thatpolymer chains of polyphosphazene are bonded to one another. The factthat the polymer is stable, that is, does not dissolve in solvents forpolyphosphazene is indictive of crosslinking.

DESCRIPTION OF A PREFERRED EMBODIMENT

This invention provides fluid separation membranes comprisingpolyphosphazene as homo- and copolymers as well as mixtures ofpolyphosphazenes as the material for polar fluid separation membranes.Fluid transport and separation properties of a variety ofpolyphosphazenes have been found to be highly selective and permeablefor polar fluids such as H₂ S and CO₂ from a gas stream containing, forexample, H₂ S, CO₂ and CH₄. Fluid dehydration applications are alsosuggested through use of halogenated polyphosphazenes, that is, theremoval of H₂ O from non-polar components either in gas or liquid form.Polyphosphazenes in general have exhibited preferential selectivitiesand permeabilities for various fluids; however, halogenated alkoxy andhalogenated aryloxy polyphosphazenes provide enhanced polar fluidrecovery beyond other polyphosphazenes discussed. For example, halogensubstituted alkoxy and aryloxy polyphosphazenes are the preferredsubstituents with most preferred being highly fluorine substitutedmaterials.

FORMATION OF POLYPHOSPHAZENES

Polyphosphazenes can be synthesized to give soluble, high molecularweight (normally greater than one million), linear chain material.Thermal polymerization of trimeric cyclo-phosphonitrylic chloridemonomer yields a high molecular weight (--PN--) skeleton which has twochlorines on each phosphorous. This poly(dichloro)phosphazene is thebase polymer from which all the soluble polyphosphazene rubbers weremade by subsequent nucleophilic displacement reactions. Typically, thesodium salt of an alcohol is used to displace chlorine on phosphorousand substitute the --O--R group in its place, as shown below: ##STR4##Copolymers are synthesized by usinq a mixture of alcoholate salts.Nitrogen rather than oxygen containing side groups linked to thephosphorous backbone atoms are made by use of amines in place of Na--ORsalts in an exchange reaction. Typically polyphosphazenes with --OR sidegroups on the phosphorous are more thermally and chemically stable forexample, to hydrolysis, than those with --NR₂ groups.

Fluid separation membranes comprised of polyphosphazenes have been foundto possess unexpectedly attractive combinations of permeability andselectivity for polar/non-polar fluid separations and indications arethat these polyphosphazenes also have significantly better thermal andchemical stability than many rubbery polymers. Fluid transportproperties and physical/chemical properties of halogenatedpolyphosphazenes indicate that this class of rubbery polymers hassignificant potential for utility in practical polar/non-polar fluidseparations on a large scale. Especially noteworthy is the CO₂ and H₂ Spermeabilities from methane containing mixed gas streams utilizinghalogenated polyphosphazenes in a dense film form. Thepoly(fluoro-alkoxy) phosphazenes produced very high permeabilities forCO₂ which were accompanied by unexpectively high CO₂ /CH₄ separationfactors found to be in the range of 10 to 12. Comparison of test resultsfor CO₂ and H₂ S transport for the poly(fluoro-alkoxy) phosphazenes withpermeability values for other gases, including hydrogen, oxygen andnitrogen clearly indicates that the polar gases as permeants exhibitvery high permeabilities in the fluoro-alkoxy substitutedpolyphosphazenes. In contrast with glassy polymers, where hydrogennormally permeates at about twice the intrinsic rate of carbon dioxide,the poly(fluoro-alkoxy)phosphazenes have carbon dioxide and hydrogensulfide permeabilities of about five times that of hydrogen.

The various polyphosphazenes utilized for gas separation membranes arerubbery materials with T_(g) well below room temperature. Thesepolyphosphazenes are soluble generally in polar organic solvents such astetrahydrofuran (THF), methanol, acetone, ethyl acetate,methylethylketone (MEK), dimethylformamide (DMF), dimethylacetamide(DMAC), formyl piperidine, N-methyl pyrrolidone and the like.Polyphosphazenes having aryl side groups are also soluble in aromatichydrocarbons, such as toluene and benzene. The latter solvents havelittle swelling impact on polyphosphazenes which have been halogenatedon the alkyl side groups. While the poly(fluoro-alkoxy)phosphazenes arereadily soluble in methanol, these phosphazenes are only sparinglysoluble in higher alcohols, for example, less than one percent inisopropyl alcohol at up to about 70° C. Various polyphosphazenes wereevaluated as gas separation membranes including those with side groupscomprised of halogenated alkoxy and halogenated aryloxy as well ascopolymers thereof, for example, poly(bis-phenoxy)phosphazene;copoly(phenoxy, p-ethyl phenoxy)phosphazene; poly(bis-trifluoroethoxy)phosphazene and the like. A copolymer of phosphazene with fluorinatedalkoxy side groups on the chain backbone of phosphorous atoms wasevaluated with results related to those results produced withpoly(bis-trifluoroethoxy)phosphazene. Overall side group composition ofthe mixed perfluorinated alkoxy copolymer, was about 65% --O--CH₂ --CF₃,about 35% --O--CH₂ --(CF₂)_(n) --CHF₂, where n is equal to 1, 3, 5, 7, 9or greater; the copolymer also containing 0.5% unsaturated functionalityin the side groups, which can be crosslinked by various vulcanizingagents such as peroxides or sulfur.

Supported polyphosphazene fluid separation membranes according to theinvention were obtained by multiple coatings onto porous filtersupports. Typically, 6 to 10% by weight tetrahydrofuran (THF) solutionof the polyphosphazenes was applied to the surface of disk ofregenerated cellulose filters (0.2 micron pore size). Vacuum was appliedto remove the solvent. Then the coating/drying procedure was repeatedtypically 3 to 7 times until a relatively thick uniform membrane of thepolyphosphazene polymer was obtained. Membrane thicknesses were obtainedby two methods: by direct micrometer measurement, subtracting theuncoated cellulose support thickness from the total thickness of thecoated support and by use of the weight gain following coating, takingthe polyphosphazene density and membrane area into account. Thicknessesobtained by the two methods agreed to within about 10%. Thickness valuesobtained from weight gain/density/area calculations were used, sincethese values would be expected to include any material which might haveimpregnated pores in the cellulose filter support. Membrane thicknesseswere typically about 0.02 cm. or lower.

Gas separation testing followed conventional procedures of Hwang, et al,employing mixed gases. Due to the high gas flux rates encountered intest of the polyphosphazene materials, most data was obtained at feedgas pressure of 10 to 20 psig. The downstream side of the test membraneswas under vacuum. Some high pressure feed tests were evaluated when themixed gas feedstream was under pressure of 100 to about 300 psig. Alltests reported were evaluated at room temperature.

Gas separation results obtained through use of four differentpolyphosphazenes as supported membranes for fluids (gases) are presentedin Table I. Separation performance of the four polyphosphazene membranesfor polar fluids, for example, CO₂ and H₂ S is illustrated by the datain Table II. Table 11 data also presents comparisons for the separatingperformance of polyphosphazenes for CH₄ and CO₂. In Table III aredisplayed comparisons of hydrogen and carbon dioxide permeability andhydrogen/methane and carbon dioxide/methane separation factors for avariety of membranes, both rubbery and glassy materials. Of particularinterest, the data illustrates unexpected positive and attractivebehavior for the polar gas permeability, and polar gas/methaneselectivity for polyphosphazenes which bear fluorinated alkoxy sidegroups. These polymers, poly(bis-trifluoroethoxy)phosphazene andcopolymers of various fluorinated alkoxy side groups, both exhibit anunusual and attractive combination of permeability (400-600 cc-cm/cm²-sec-cmHg) and selectivity (10-11.5) properties for polar gasseparations from methane. Such high carbon dioxide permeabilties areexceeded only by silicon-based polymers. For example,polydimethylsiloxane is reported having as high as 3,000 to 4,000standard units for carbon dioxide permeability; however, the separationfactors for carbon dioxide/methane are considerably lower, for example,3 to 4. While many glassy polymers exhibit higher carbon dioxide/methaneselectivities [about 30 for polysulfone and 35-50 for ammoniacrosslinked brominated polyphenylene oxide (PPO, for example,2,6-dimethyl-1,4-phenylene oxide)], CO₂ permeability is typically lowerby one or two orders of magnitude, for example, about 6 for polysulfoneand about 40 to 45 for the crosslinked brominated PPO membranes.Measurements indicate that hydrogen sulfide permeability for thefluoro-alkoxy substituted polphosphazenes is about equal to that ofcarbon dioxide.

                                      TABLE I                                     __________________________________________________________________________    Fluid Gas Transport Properties of Polyphosphazenes                            (H.sub.2, CH.sub.4, CO, O.sub.2, N.sub.2)                                                        .sup.P H.sub.2                                             Membrane                                                                            Polymer      (× 10.sup.-10)                                                                α .sub.CH.sbsb.4.sup.H.sbsp.2                                               α .sub.CO.sup.H.sbsp.2                                                      (× 10.sup.-10)                                                                α .sub.N.sbsb.2 .sup.O.sbsp.2    __________________________________________________________________________    1     Poly(bis-trifluoroethoxy)                                                                   90.-100.                                                                           1.6-1.9                                                                           2.0-2.1                                                                           83.-89.                                                                             2.0-2.1                                      phosphazene                                                             2.    Copoly(fluoro-alkoxy)                                                                      80.-90.                                                                             1.3 2.0 72.   1.7                                          phosphazene                                                             3.    Poly(bis-diphenoxy)                                                                        9.    2.4 --   4.   3.6                                          phosphazene                                                             4.    Copoly       12.-14.                                                                             2.4-2.5                                                                           5.  5.4-5.6                                                                             3.4                                          (phenoxy, p-ethylphenoxy)                                                     phosphazene                                                             __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Polar Fluid Transport Properties of Polyphosphazenes                          CH.sub.4, CO.sub.2, H.sub.2 S                                                                    .sup.P CO.sub.2                                                                          .sup.P H.sub.2 S                                Membrane                                                                            Polymer      (× 10.sup.-10)                                                                α .sub.CH.sbsb.4.sup.CO.sbsp.2                                               (× 10.sup.-10)                                                                α .sub.H.sbsb.2.sub.S.sup.CO.sbs                                        p. 2                                      __________________________________________________________________________    1     Poly(bis-trifluoroethoxy)                                                                  550.-600.                                                                            10.-11.3                                                                          550.  1.                                              phosphazene                                                             2.    Copoly(fluoro-alkoxy)                                                                      400.   11.-11.5                                                                          --    --                                              phosphazene                                                             3.    Poly(bis-diphenoxy)                                                                         76.  6.6-7.0                                                                            --    --                                              phosphazene                                                             4.    Copoly       36.-39.                                                                             6.4-6.9                                                                            --    --                                              (phenoxy, p-ethylphenoxy)                                                     phosphazene                                                             __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    Comparison of Fluid Transport Properties for Various Glassy and Rubbery       Polymers                                                                      Calculated                                                                                        .sup.P H.sub.2                                                                           .sup.P CO.sub.2                                Membrane                                                                            Rubbery Polymers                                                                            (× 10.sup.-10)                                                                α .sub.CH.sbsb.4.sup.H.sbsp.2                                                (× 10.sup.-10)                                                                α .sub.CH.sbsb.4 .sup.CO.sbsp.2                                         7    α .sub.H.sbsb.2.sup.CO.sbs                                              p.2                                 __________________________________________________________________________    1.    Polydimethylsiloxane                                                                        650.   0.68                                                                              3,250.                                                                              3.4  5.                                  2.    Poly          100.  1.8  550.-600.                                                                           10.-11.                                                                            5.5-6.                                    (bis-trifluoroethoxy)                                                         phosphazene                                                             3.    Copoly(fluoro-alkoxy)                                                                       85.   1.3  400.   11.-11.5                                                                          4.7                                       phosphazene                                                             4.    Polychloroprene                                                                             14.   --    26.  --   1.8                                 5.    Polyisoprene  51.   1.8  134.  4.6  2.6                                       (density 0.9689)                                                        6.    Poly(bis-phenoxy)                                                                            9.   2.4   76.  6.6-7.0                                                                            8.4                                       phosphazene                                                             7.    Poly(dimethyl,                                                                              100.-120.                                                                           30.   75.  18.-21.                                                                            0.63-0.75                                 p,phenylene oxide), PPO                                                 8.    Copoly(phenoxy,                                                                             12.-14.                                                                             2.4-2.5                                                                            36.-39.                                                                             6.4-6.9                                                                            2.5-3.3                                   p-ethylphenoxy)                                                               phosphazene                                                             9.    Untreated Borminated PPO                                                                    22.   53.-55.                                                                            8.-9. 21.-22.                                                                            0.36-0.41                                 Amino Cross-linked                                                                          110.  34.   42.  35.   0.38                                     Brominated PPO                                                                Amino Cross-linked                                                                          23.   152.  9.   47.   0.39                                     Brominated PPO +                                                              HBr Treatment                                                           10.   Cellulose triacetate                                                                        10.-11.                                                                             80.  5.-6. 30.-33.                                                                            0.45-0.6                            11.   Polysulfone    8.-10.                                                                             60.-70.                                                                            4.-6. 27.-31.                                                                            0.4-0.6                             __________________________________________________________________________

It appears that polyphosphazene membranes interact in some unusualmanner with polar fluids to account for the transport behaviorsobserved. In particular, the poly(fluoro-alkoxy) phosphazenes mustinteract in some manner with CO₂ and H₂ S. An additional experiment wasmade by using a three-component mixed gas feed of H₂ S/CO₂ /CH₄ at 20psig for a membrane of poly(bis-trifluoroethoxy)phosphazene supported ona porous polypropylene filter (0.02 micron pore). The results indicatedthat CO₂ /CH₄ permeation behavior was very similar to that observedpreviously in the absence of H₂ S, indicating generally that themembrane itself was intact. However the H₂ S/CO₂ separation factor isessentially unity. Thus, while the poly-(fluoro-alkoxy)phosphazenes holdlittle attraction for separation of polar gas pairs, such as H₂ S/CO₂they find utility in gas mixtures where the desired separation is thatof CO₂ or H₂ S from gaseous hydrocarbons or other non-polar gases.

The permeability of CO₂ in poly(fluoro-alkoxy)phosphazenes wassignificantly higher than the permeabilities for the same membrane forhydrogen. Although mixed gases such as CO₂ /hydrogen tests have not beenmade, the present data suggests that CO₀₂ /hydrogen separation factorsfor poly(bis-trifluoroethoxy)phosphazenes and various phosphazenemixtures would be about 5 to 6. Generally, in the case of glassypolymers hydrogen permeability is observed to be roughly twice that ofcarbon dioxide. In some rubbery polymers, carbon dioxide permeatesfaster than hydrogen, for example, silicon rubber has been reported tohave intrinsic permeabilities for CO₂ and H₂ of 3200 and 660 standardunits respectively. These values for silicones suggest CO₂ /H₂separation factor of about 5, comparable to the above estimates for thepoly(fluoro-alkoxy)phosphazenes. Thus, the poly(halo-alkoxy)phosphazenesmay find utility in separations of gas pairs which, at least in the caseof most glassy polymers, would be termed fast gases. For example,potential value may exist in situations where it is desirable to affectseparation of polar gases, i.e. CO₂ and H₂ S from H₂.

Not included in the three tables is a fluid separation example whereinthe separation of a mixture of liquid toluene and cyclohexane wasaccomplished through the use of a trifluoroethoxy phosphazene membraneat ambient temperature and at 24 psig. The permeate was enriched intoluene vs. the toluene content in the feedstream as follows: Toluene tocyclohexane molar ratio and the permeate was 1.43 vs. 1.23 in the liquidfeed mixture.

Included in Table III are calculated values for CO₂ /H₂ separationfactors, based solely on ratios of CO₂ and H₂ permeability valuesobtained from CO₂ /methane and H₂ /methane mixed gas test.

The ability to make practical use of the attractive intrinsic transportproperties for polar gases using polyhalogenated alkoxy phosphazenes, orany other rubbery polymer in the form of fluid separation membranes,would at first sight seem limited largely by the poor mechanicalproperties of the rubbery state compared to the high mechanical strengthtypically found for glassy polymers. However, technology exists foreffectively supporting thin rubbery polymer membranes in configurationssuitable for fluid separations. For example, ultra-thin approximately500 A silicone based rubbery membranes supported atop porous supportshave been fabricated into small scale medical devices suitable for bloodoxygenation or production of oxygen enriched air. Studies of thepolyphosphazenes as fluid separation membranes have lead to thepossibility of effectively supporting very thin separation membranes ofa wide variety of polymers, including rubbery materials such aspolyphosphazenes atop microporous supports in hollow fiberconfigurations. For example, experimentation at supportingpoly(bis-trifluoroethoxy) phosphazene atop mircroporous polypropylenehollow fibers has yielded composite fiber membranes approaching theintrinsic separation factor of the polymer for CO₂ /CH₄ (8.3 versusintrinsic 10.5). This result indicates that very nearly completeintegrity of the polyphosphazene separating layer was achieved.Permeability, (P/1), of CO₂ of this particular sample was 83(P/1) bycomparison to the intrinsic permeability of CO₂ of aboul 600(P) for theparticular polyphosphazene; thus allowing the estimate that thethickness of the supported polyphosphazene membrane as 7.5 microns. At athickness at about 2 microns, the P/1 of CO₂ could be obtained at aboutthe 300 standard unit level or above.

Apart from the attractive transport properties of the polyhaloalkoxypolyphosphazene described above, these materials also possess highthermal and chemical stability, which may broaden their utilization invarious gas separation applications. For example, differential scanningcalorimetry of these polyphosphazenes over a temperature extending from-100° to 300° C., has shown no thermal activity other than the glass torubber phase transition T_(g) which is observed near -60° C. Thesepolyphosphazene materials possess substantial resistance to degradationby common solvents and other organics which may be encountered in somegas separation applications. For example, at room temperature in liquidtoluene, silicone rubber swells 140 volume percent while apolyphosphazene mixture swells only about 15%. After immersion at 100°C. for seven days in JP4 jet fuel (kerosene fraction hydrocarbons) thepolyphosphazene mixture had swelled only about 9% of volume; thus theearlier interest in polyphosphazenes in the area of specialty 0-ring andgasket applications.

Further examples of the unique stability features of polyphosphazenematerials are found in the comparison of the hot tensile strength ofpolyphosphazene mixtures to that of fluorosilicone elastomers. Whilefluorosilicone elastomers exhibit a relatively low equilibrium swellingupon immersion in liquid toluene at room temperature (about 20% versusabout 15% for polyphosphazenes), tensile strength retention of thepolyphosphazenes is considerably better at elevated temperatures, forexample, equal to or greater than 50% retention at 150° C. for thepolyphosphazenes versus only about 25% retention for the fluorosilicone.It is expected that the polyphosphazene gas separation membranes wouldhave an upper limit to prolonged exposure to temperatures of up to about175° C.; however, these upper temperatures are beyond the upper limit ofsurface temperatures generally contemplated either in gas separationmembrane fabrication or gas separation applications.

As attractive as the above described transport and physical/chemicalproperties of the polyphosphazenes are, the ability to crosslink theunsaturated fluorinated polyphosphazenes further enhances the polymers'potential utility in fluid separations. Thus, crosslinkedpolyphosphazenes would provide applications involving aggressive useenvironments e.g., feedstreams containing solvating components orswelling impurity components or elevated temperature environments. Thecrosslinking of various poly(fluoro-alkoxy)phosphazenes may beaccomplished to various extents as may be desired, by treatment of thepolyphosphazene with, for example, the disodium salt of highlyfluorinated alkyl diols. Such crosslinking reactions in solution may bereadily accomplished under mild room temperature conditions and occur toan extent closely related to the stoichiometric ratio of reactants.Crosslinking may also be performed on polyphosphazene in the solidstate. In such case, higher temperatures are normally required comparedto solution crosslinking. Crosslinking, as described above by thedisplacement-exchange reaction effected with fluorinated disodiumdialkoxides, can be carried out in much higher crosslink densities thancould crosslinking reactions employing peroxide or other free radicalagents wherein the starting polyphosphazene materials have small amountsof unsaturated sites. Thus, with the information available and asderived from present testing, polyphosphazenes have sufficient physicalproperties for potential utility in a variety of fluid separationapplications. Potential uses include separations of the polar gases suchas CO₂ and H₂ S from H₂ and slow gases, particularly CH₄. For suchseparation applications the transport properties of polyphosphazenesappear to offer distinct advantages in terms of their combination ofpermeability and selectivity as compared to various rubbery and glassypolymers.

Permeability of gases of rubbery polymers, unlike that in glassypolymers, is essentially independent of pressure. This follows from therelationship: P=D×S, where P, D and S are the permeability, diffusivityand solubility coefficients for the gas in the polymer. Gas solubilityin rubbery polymers follows Henry's Law, thus the solubilitycoefficients are pressure independent, as is the diffusivity coefficientD. Therefore gas transport properties at high pressures should not betoo different from those at low pressures. ln order to minimizeperformance degradation, operation at elevated temperatures isfrequently employed to raise the saturation vapor pressure of harmfulcontaminants sufficiently so as to lower the relative saturation levelas much as practical. Conceivably due to the high permeabilities, andgood chemical resistance of polyphosphazenes as gas separationmembranes, high temperature operation would not be necessary. In such acase, the effectiveness of the separation would depend on the adequacyof chemical resistance exhibited by the porous support in the case of acomposite membrane utilizing polyphosphazenes as the coating.

The supported polyphosphazene fluid separation membranes as indicated inTable IV illustrating water transport properties, were preparedaccording to the same procedures as those membranes of Tables I, II andIII. Separation factors for water/methane were calculated values asindicated in Table IV using the ratio of measured water permeability,P(H₂ O) and separately measured methane permeability, P(CH₄). Fluidseparation testing followed conventional procedures. Waterpermeabilities P(H₂ O), were measured with liquid water on the upstreamside of the test membranes. The downstream side of the test membraneswas under vacuum. All tests reported were evaluated at room temperature.

                  TABLE IV                                                        ______________________________________                                        Water Transport Properties of Polyphosphazenes                                Calculated                                                                    Mem-                      .sup.P H.sub.2 O                                    brane Polymer             (× 10.sup.-10)                                                                    α .sub.CH.sbsb.4 .sup.H.sbsp.2.s                                        up.O                                      ______________________________________                                        1     Poly(bis-trifluoroethoxy)                                                                         4,600-5,6000                                                                             90-100                                         phosphazene         (5,100)   (95)                                      2     Copoly(fluoro-alkoxy)                                                                             2,600-3,600                                                                             80-90                                           phosphazene         (3,100)   (85)                                      3     Poly(bis-diphenoxy) 800.sup.a 800.sup.b                                                                     69-74                                           phosphazene                                                             4     Copoly(phenoxy,p-ethylphen-                                                                       324       53-62                                           oxy)-phosphazene                                                        5     Copoly(phenoxy,p-isopropyl-                                                                       285       21                                              phenoxy)-phosphazene                                                    ______________________________________                                         .sup.a Chattopadhyay et al. Journal of Coating Technology, 51 (658), 87       (1979) H.sub.2 O permeability values obtained using ASTM (D1653 and E96)      test procedures.                                                              .sup.b Water permeability values obtained using test procedures as            indicated for Tables I-III.                                              

The membranes of Table IV clearly illustrate the superior permeabilitiesof water when utilizing preferred halogenated polyphosphazenes accordingto the invention. The halogenated polyphosphazenes achieved from 30 to70 times the water permeability rates as compared to non-halogenatedpolyphosphazenes indicated by membrane number 3. These unexpected andexceptional permeabilities for water are also accompanied by reasonableseparation factors as indicated by the calculated alpha column of thetable.

In addition, a fluid separation membrane exhibiting relativepreferential selectivity and permeabilities for at least one fluidpermeate from a mixture of fluids comprising a polyphosphazenerepresented by the formula: ##STR5## where n is between about 100 andabout 70,000; where Y and Y' are the same or different and are comprisedof oxygen, sulfur or nitrogen; R and R' are the same or differentaccording to the invention; the polyphosphazene being comprised ofpoly(bis-anilino)phosphazene, which was prepared in the same fashion asthe membranes of Tables I through IV. This phosphazene according to theinventive formula presents a nitrogen atom in the Y and Y' position. Thepolymer achieved a P_(O).sbsb.2 (10⁻¹⁰) of 10 and an alpha of O₂ /N₂ of1.1; and a P_(CO).sbsb.2 (10⁻¹⁰) of 64 and an alpha for CO₂ /CH₄ of 1.34to 1.76.

We claim:
 1. A process for separating polar fluids from non-polar fluids comprising:contacting a fluid feed mixture of polar and non-polar fluids with a first surface of a separation membrane comprising polyphosphazenes having a preferential selectivity and permeability for the polar fluids and represented by the formula: ##STR6## where n is an integer of from about 100 to about 70,000 and ORX and OR'X' each represent a halogenated alkoxy or halogenated aryloxy where R and R' can be the same or different with from 1 to about 25 carbon atoms and X and X' can be the same or different halogens; maintaining a second surface of the polyphosphazene membrane at a lower chemical potential for the polar fluids than the chemical potential at the first surface; permeating the polar fluids into and through the polyphosphazene membrane; and removing from the vicinity of the second surface a permeated product having a greater concentration of polar fluids relative to non-polar fluids than contained in the feed mixture. 